In the treatment of sewage sludge, belt presses are commonly employed in the removal of water from the processed sewage sludge so as to permit separation of solid material and water. The solid materials having a high water content are fed to a belt press where this material is compressed between a pair of fabric or mesh belts and the material is compressed between the belts as the belts pass between a plurality of pairs of rollers. As the material is compressed, the water is forced through the permeable mesh belts. The belt presses thus produce a solid cake of material which can then be delivered to additional apparatus for further processing such as drying or disposal.
One of the problems with the prior art apparatus for use in removing water from sludge has been that the costs of manufacture or construction of the belt presses place limits on the capability of the presses to handle large quantities of sewage sludge. If the belt presses are required to handle large quantities of sludge, it is necessary that the belt presses employ relatively wide belts. Use of wide belts requires the construction of relatively long rollers to support the belts. During the operation of the belt press the rollers are subjected to substantial forces and bending moments when they apply compressive force on the belt. With the employment of relatively long rollers in high capacity belt presses, the rollers are subjected to substantial moments, and accordingly, the rollers must have substantial strength and must be supported by bearings capable of withstanding the forces applied by the rollers. Due to the cost of construction of oelt presses having such elongated rollers, it is preferred that the presses employ relatively narrow belts and rollers of reduced length.
Prior art apparatus for use in treating sewage sludge also includes free drain decks or gravity belt thickeners. The prior art gravity belt thickeners include a mesh belt supported by a pair of horizontally spaced apart rollers. The mesh belt includes an elongated horizontal upper belt flight. Sewage sludge having a relatively low solids content is deposited onto one end of the belt and is carried by the belt toward the opposite end of the gravity belt thickener. Water in the sludge drains through the mesh belt, and accordingly the sludge at the discharge end of the belt will have a higher solids content.
Prior art methods and apparatus for use in processing sewage sludge also commonly include the step of mixing a polymer material, such as that described in Great Britain Pat. No. 1,602,677, with the sewage sludge to cause flocculation of the solid materials in the sludge. The formation of flocculant is desired in order to permit increased drainage of the water away from the solid materials during the de-watering or removal of the water from the solid materials. The addition of the polymer material to form the flocculant or floc has the effect of producing a product which is relatively fragile. Rough handling of the floc material causes the floc to break down and inhibits drainage of water from the solids. Once the floc has been handled roughly and broken down, it is then necessary to add additional polymer in order to reform or regenerate the floc and thereby requiring overdosing of polymer. It will be readily understood that the addition of polymer to the liquid sludge results in expense, and repetition of this step of introducing polymer to the sludge should be avoided if possible. Accordingly, it is preferred that the sludge be processed soon after the reaction of polymer with the sludge in such a manner that the water can be removed from the sludge or floc in a single step and the flock is also to be handled in a relatively gentle manner to prevent the floc from breaking down before the removal of the water.